The Blind Watchmaker (48 page)

Cladists would look at each of the three possible trees in turn, and choose the best tree. How is the best tree recognized? Basically, it is the tree that unites the animals that have the most features in common. We label as the ‘outgroup’ the animal that has fewest features in common with the other two. Of the above list of trees, the second would be preferred, because human and herring share many more features in common with each other than squid and herring do or than squid and human do. Squid is the outgroup because it doesn’t have many features in common with either human or herring.

Actually, it isn’t quite as simple as just counting features in common, because some kinds of features are deliberately ignored. Cladists want to give special weight to features that are recently evolved. Ancient features that all mammals inherited from the first mammal, for instance, are useless for doing classifications within the mammals. The methods they use for deciding which features are ancient are interesting, but they would take us outside the scope of this book. The main thing to remember at this stage is that, at least in principle, the cladist thinks about all possible bifurcating trees that
might
unite the set of animals he is dealing with, and tries to choose the one correct tree. And the true cladist makes no bones about the fact that he thinks of the branching trees or ‘cladograms’ as family trees, trees of closeness of evolutionary cousinship.

If pushed to the extreme, the obsession with branchings alone could give strange results. It is theoretically possible for a species to be
identical
in every detail to its distant cousins, while being exceedingly different from its closer cousins. For instance, suppose that two very similar fish species, which we can call Jacob and Esau, lived 300 million years ago. Both these species founded dynasties of descendants, which last to the present day. Esau’s descendants stagnated. They went on living in the deep sea but they didn’t evolve. The result is that a modem descendant of Esau is essentially the same as Esau, and is therefore also very like Jacob. Jacob’s descendants evolved and proliferated. They eventually gave rise to all modem mammals. But one lineage of Jacob’s descendants also stagnated in the deep sea, and also leaves modern descendants. These modern descendants are fish that are so similar to Esau’s modern descendants that they are hard to tell apart.

Now, how shall we classify these animals? The traditional evolutionary taxonomist would recognize the great similarity between the primitive deep-sea descendants of Jacob and Esau, and would classify them together. The strict cladist could not do that. The deep-sea descendants of Jacob, for all that they look just like the deep-sea descendants of Esau, are, nevertheless, closer cousins of mammals. Their common ancestor with mammals lived more recently, even if only slightly more recently, than their common ancestor with Esau’s descendants. Therefore they must be classified together with mammals. This may seem strange, but personally I can treat it with equanimity. It is at least utterly logical and clear. There are indeed virtues in both cladism and traditional evolutionary taxonomy, and I don’t much mind how people classify animals so long as they tell me clearly how they are doing it.

Turning now to the other major school of thought, the pure-resemblance measurers, they again can be split into two subschools. Both the subschools agree to banish evolution from their day-to-day thoughts while they do taxonomy. But they disagree over how to proceed in their day-to-day taxonomy. One subschool among these taxonomists are sometimes called ‘pheneticists’ and sometimes called ‘numerical taxonomists’. I shall call them the ‘averagedistance measurers’. The other school of resemblance measurers call themselves ‘transformed cladists’. This is a poor name, since the one thing these people are not is cladists! When Julian Huxley invented the term clade he defined it, clearly and unambiguously, in terms of evolutionary branching and evolutionary ancestry. A clade is the set of all organisms descended from a particular ancestor. Since the main point of ‘transformed cladists’ is to avoid all notions of evolution and of ancestry, they cannot sensibly call themselves cladists. The reason they do so is one of history: they started out as true cladists, and kept some of the methods of cladists while abandoning their fundamental philosophy and rationale. I suppose I have no choice but to call them transformed cladists, although I do so with reluctance.

The averagedistance measurers not only refuse to use evolution in their taxonomy (although they all believe in evolution). They are consistent in that they don’t even assume that the pattern of resemblance will necessarily be a simply branching hierarchy. They try to employ methods that will uncover a hierarchical pattern if one is really there, but not if it isn’t. They try to ask Nature to tell them whether she is really organized hierarchically. This is not an easy task, and it is probably fair to say that methods are not really available for achieving this aim. Nevertheless the aim seems to me to be all of a piece with the laudable one of avoiding preconceptions. Their methods are often rather sophisticated and mathematical, and they are just as suitable for classifying nonliving things, for instance rocks or archaeological relics,’ as for classifying living organisms.

They usually begin by measuring everything they can about their animals. You have to be a bit clever about how you interpret these measurements, but I shan’t go into that. The end result is that the measurements are all combined together to produce an index of resemblance (or, its opposite, an index of difference) between each animal and each other animal. If you wish, you can actually visualize the animals as clouds of points in space. Rats, mice, hamsters,
etc.
would all be found in one part of the space. Far away in another part of the space would be another little cloud, consisting of lions, tigers, leopards, cheetahs,
etc.
The distance between any two points in the space is a measure of how closely the two animals resemble each other, when large numbers of their attributes are combined together. The distance between lion and tiger is small. So is the distance between rat and mouse. But the distance between rat and tiger, or mouse and lion, is large. The combining together of attributes is usually done with the aid of a computer. The space that these animals are sitting in is superficially a bit like Biomorph Land, but the ‘distances’ reflect bodily resemblances rather than genetic resemblances.

Having calculated an index of average resemblance (or distance) between each animal and each other animal, the computer is next programmed to scan the set of distances\resemblances and to try to fit them into a hierarchical clustering pattern. Unfortunately there is a lot of-controversy about exactly which calculation method should be used to look for clusters. There is no one obviously correct method, and the methods don’t all give the same answer. Worse, it is possible that some of these computer methods are over-‘eager’ to ‘see’ hierarchically arranged clusters within clusters, even if they aren’t really there. The school of distance measurers, or ‘numerical taxonomists’, has become a bit unfashionable lately. My view is that the unfashionableness is a temporary phase, as fashions often are, and that this kind of ‘numerical taxonomy’ is by no means easily to be written off. I expect a comeback.

The other school of pure-pattern measurers are the ones that call themselves transformed cladists, for reasons of history as we have seen. It is from within this group that the ‘nastiness’ mainly emanates. I shall not follow the usual practice of tracing their historical origins from within the ranks of true cladists. In their underlying philosophy, so-called transformed cladists have more in common with the other school of pure-pattern measurers, the ones often called ‘pheneticists’ or ‘numerical taxonomists’, whom I have just discussed under the title of averagedistance measurers. What these share with each other is an antipathy to dragging evolution into the practice of taxonomy, although this does not
necessarily
betoken any hostility to the idea of evolution itself.

What the transformed cladists share with true cladists is many of their methods in practice. Both think, right from the start, in terms of bifurcating trees. And both pick out certain kinds of characteristics as taxonomically important, other kinds of characteristics as taxonomically worthless. They differ with respect to the rationale that they give to this discrimination. Like averagedistance measurers, transformed cladists are not out to discover family trees. They are looking for trees of pure resemblance. They agree with the averagedistance measurers to leave open the question of whether the pattern of resemblance reflects evolutionary history. But unlike the distance measurers, who, at least in theory, are prepared to let Nature tell them whether she is actually hierarchically organized, the transformed cladists
assume
that she is. It is an axiom, an article of faith with them, that things are to be classified into branching hierarchies (or, equivalently, into nested nests). Because the branching tree has nothing to do with evolution, it need not necessarily be applied to living things. The methods of transformed cladistics can, according to their advocates, be used for classifying not just animals and plants but stones, planets, library books and Bronze Age pots. In other words they would not subscribe to the point I made with my library comparison, that evolution is the only sound basis for a uniquely hierarchical classification.

The averagedistance measurers, as we saw, measure how far each animal is from each other animal, where ‘far’ means ‘does not resemble’ and ‘near’ means ‘resembles’. Only then, after calculating a sort of summed average index of resemblance, do they start trying to interpret their results in terms of a branching, cluster-within-clustery hierarchy or ‘tree’ diagram. The transformed cladists, however, like the true cladists that they once were, bring in clustery, branchy thinking right at the outset. Like true cladists, they would begin, at least in principle, by writing down all possible bifurcating trees, and then choosing the best.

But what are they actually talking about when they consider each possible ‘tree’, and what do they mean by the best? What hypothetical state of the world does each tree correspond to? To a true cladist, a follower of W.Hennig, the answer is very clear. Each of the 15 possible trees uniting four animals represents a possible family tree. Of all the 15 conceivable family trees uniting four animals, one and only one must be the correct one. The history of the animals’ ancestors really did happen, in the world. There are 15 possible histories, if we make the assumption that all branchings are two-way branchings. Fourteen of those possible histories must be wrong. Only one can be right; can correspond to the way the history actually happened. Of all the 135,135 possible family trees culminating in 8 animals, 135,134 must be wrong. Only one represents historical truth. It may not be easy to be sure
which
one is the correct one, but the true cladist can at least be sure
that
not more than one is correct.

But what do the 15 (or 135,135, or whatever it is) possible trees, and the one correct tree, correspond to in the nonevolutionary world of the transformed cladist? The answer, as my colleague and former student Mark Ridley has pointed out in his
Evolution and Classification
, is nothing very much. The transformed cladist refuses to allow the concept of
ancestry to
enter his considerations. ‘Ancestor’, to him, is a dirty word. But on the other hand he insists that classification must be a branching hierarchy. So, if the 15 (or 135,135) possible hierarchical trees are not trees of ancestral history, what on earth are they? There is nothing for it but to appeal to ancient philosophy for some woolly, idealistic notion that the world just is organized hierarchically; some notion that everything in the world has its ‘opposite’, its mystical ying or yang. It never gets much more concrete than that. It certainly is not possible, in the nonevolutionary world of the transformed cladist, to make strong and clear statements such as ‘only one out of the 945 possible trees uniting 6 animals can be right; all the rest must be wrong’.

Why is ancestor a dirty word to cladists? It is not (I hope) that they think that there never were any ancestors. It is rather that they have decided that ancestors have no place in taxonomy. This is a defensible position as far as the day-to-day
practice
of taxonomy is concerned. No cladist actually draws flesh and blood ancestors on family trees, though traditional evolutionary taxonomists sometimes do. Cladists, of all stripes, treat all relationships between real, observed animals as
cousinships
, as a matter of form. This is perfectly sensible. What is not sensible is to carry this over into a taboo against the very
concept
of ancestors, against the use of the language of ancestry in providing the fundamental justification for adopting the hierarchically branching tree as the basis for your taxonomy.

I have left till last the oddest aspect of the transformed cladism school of taxonomy. Not content with a perfectly sensible belief that there is something to be said for leaving evolutionary and ancestral assumptions out of the
practice
of taxonomy, a belief that they share with pheneticist ‘distance measurers’, some transformed cladists have gone right over the top and concluded that there must be something wrong with evolution itself! The fact is almost too bizarre to credit, but some of the leading ‘transformed cladists’ profess an actual hostility to the idea of evolution itself, especially the Darwinian theory of evolution. Two of them, G.Nelson and N.Platnick from the American Museum of Natural History in New York, have gone so far as to write that ‘Darwinism … is, in short, a theory that has been put to the test and found false’. I should love to know what this ‘test’ is and, even more, I should love to know by what alternative theory Nelson and Platnick would explain the phenomena that Darwinism explains, especially adaptive complexity.

It isn’t that any transformed cladists are themselves fundamentalist creationists. My own interpretation is that they enjoy an exaggerated idea of the importance of taxonomy in biology. They have decided, perhaps rightly, that they can do taxonomy better if they forget about evolution, and especially if they never use the concept of the ancestor in thinking about taxonomy. In the same way, a student of, say, nerve cells, might decide that he is not aided by thinking about evolution. The nerve specialist agrees that his nerve cells are the products of evolution, but he does not need to use this fact in his research. He needs to know a lot about physics and chemistry, but he believes that Darwinism is irrelevant to his day-to-day research on nerve impulses. This is a defensible position. But you can’t reasonably say that, because you don’t need to use a particular theory in the day to day practice of your particular branch of science, therefore that theory is
false
. You will only say this if you have a remarkably grandiose estimation of the importance of your own branch of science.